1. Field of the Invention
The present invention relates to an improved apparatus for moving a die or a mandrel to form a bent tubular portion in a pipe making machine.
2. Description of Related Art
In a known method for manufacturing a bent pipe, a material in a fluid state, such as a synthetic resin or the like, is extruded to be molded by an extruder to form a straight tubular blank. The blank is then introduced onto a core bar which has a predetermined curved profile and which has a diameter substantially equal to the inner diameter of the straight tubular blank, to reform the same. After that, the reformed blank is withdrawn from the core bar to produce a product. An inflation process is also known in which the tubular blank is introduced in a predetermined shape of a die assembly having an outer die and an intermediate die under pressure to reform the tubular blank, so that the intermediate die is withdrawn to produce a product. Or, the blank is introduced only by the use of the outer die, so that pressurized fluid (air, gas or liquid, etc.) is enclosed in the die to mold the product into a shape corresponding to the outer die.
However, in these known methods, there is a large number of manufacturing steps, including manual operations, that result in decreased operability. Furthermore, since the straight blank is bent to form the bent portion, the outer portion thereof is thinner, and as a result, the thin outer portion tends to be broken by the pressurized fluid. In addition, due to the aging of the core bars and the die assembly, the quality of the products becomes irregular. Furthermore, the manufacturing cost of the dies increases and this leads to expensive products.
The assignee of the present application has proposed a method and apparatus for manufacturing a bent portion of a pipe in Japanese Unexamined Patent Publication No. 59-123636.
In this manufacturing method and apparatus, a fluid material is extruded to form a tubular body, a die or a mandrel is moved in a lateral direction with respect to the direction of extrusion to form the gap between the die and the mandrel to be uniform or not uniform throughout the circumference to thereby form a straight or bent tubular portion.
In principle, any three-dimensional shape of bent tubular body can be obtained by moving the die or the mandrel in accordance with the angles defining the three-dimensional shape. However, in the apparatus disclosed in JUPP 59-123636 mentioned above, one of the die and the mandrel is moved by an external force to vary the gap between the die and the mandrel. Thus, constituent elements of the die and the mandrel or supporting members supporting the constituent elements must be interconnected or fastened with a strong press to prevent possible leakage of the fluid into spaces between the constituent elements or the supporting members thereof. Consequently, the movement of the die or the mandrel requires a very large external drive force, in which case a large driving device with high power and high strength would be required, which is impracticable. Furthermore, a possible backlash of a transmission mechanism of the drive force makes it difficult to precisely control the dimension of the gap between the die and the mandrel, which makes the formation of a desired shape of a bent pipe difficult.